Solid carbon dioxide in the form of solid flakes or "snow" is formed by passing liquid carbon dioxide through a narrow orifice under a significant pressure drop resulting a sudden, drastic reduction in temperature and in the formation of solid carbon dioxide flakes or snow.
Solid carbon dioxide snow is a desirable substance for the chilling and/or freezing of foodstuffs. Solid carbon dioxide flakes exist at a temperature of approximately -109 F. and therefore can provide rapid cooling and/or freezing of seafood, fish, poultry, meats, vegetables and the like. In addition, the solid carbon dioxide flakes vaporize when heated thereby leaving no residue on the foodstuffs. For these reasons, as well as its relatively low cost, solid carbon dioxide is an effective heat exchange medium for use in commercial freezers requiring a high throughput. In addition, solid carbon dioxide flakes prevent pieces of foodstuffs from sticking to each other. The flakes sublime on the surfaces of foodstuffs while freezing the moisture on the surfaces thus preventing the pieces of foodstuffs from freezing together. Similarly, the flakes prevent foodstuffs from adhering to the conveyor belt as it moves through the refrigeration process.
Solid carbon dioxide flakes are typically produced by taking liquid carbon dioxide which exists at high pressures (e.g. 300 psi) and passing the same through a restricted orifice or nozzle. The temperature of the liquid carbon dioxide under such pressures is typically about 0 F. However, the passing of the liquid carbon dioxide through a restricted orifice results in a rapid and significant pressure drop and a corresponding rapid and sudden decrease in temperature to about -109 F.
The solid carbon dioxide is typically applied to foodstuffs in commercial refrigeration systems either directly from the nozzle or from a nozzle which has attached thereto a tube for directing the flow of the solid carbon dioxide flakes to a substrate such as foodstuffs on a moving conveyer belt.
Each of these systems for directing solid carbon dioxide flakes to a moving conveyor belt often does not provide a uniform coating across the width of the conveyor belt of the solid carbon dioxide flakes over the foodstuffs. With regard to the employment of a nozzle assembly alone, the sudden reduction in pressure and the concurrent formation of solid carbon dioxide flakes results in a forceful and non-uniform spray of the flakes over a wide area. The nozzle assembly which is typically comprised of a single nozzle directs the flakes at an angle to the moving conveyor or belt such that a portion of the flakes miss the surface of the belt and/or are concentrated in one region of the belt. In particular, the resulting coating profile typically provides greater deposition of solid carbon dioxide flakes in the center of the spray region as opposed to the outer fringes of the spray region.
This problem can, to some extent, be corrected by the use of a tube surrounding the nozzle head and extending to the proximity of the substrate. The tube prevents the solid carbon dioxide flakes from traveling over a wide region and therefore can provide a more uniform coating profile than the nozzle alone. However, the walls of the tube become cooled by the solid carbon dioxide flakes to such an extent that the solid carbon dioxide flakes begin to adhere to the walls of the tube. This occurs especially when there is no significant difference in temperature between the tube walls and the surrounding environment such as when the tubes are within the refrigeration system. This causes the formation of agglomerates or lumps of solid carbon dioxide flakes which restricts the flow thereof and can in some circumstances actually cut off the flow of the solid carbon dioxide flakes to the substrate. When this happens, it is necessary to shut down the solid carbon dioxide flake generator and clean the tubes which results in lost production time and higher cost of cooling/freezing.
It would therefore be a significant advance in the art of employing solid carbon dioxide flakes for cooling and/or freezing of foodstuffs to be able to apply the same in a uniform manner. It would be a further advance in the art to be able to apply the solid carbon dioxide flakes in a uniform manner in a system which can run continuously without frequent shut downs to remove agglomerations of the solid carbon dioxide flakes.